The present invention is with respect to a driver seat, more specially for tractors and other vehicles under the control of a driver, having an upper chair-like part with a seat plate and a backrest and which is guided in relation to a support or base-frame by way of a system with crossing levers so that the upper chair part is guided on being moved vertically, its weight being supported on the base-frame by way of a spring acting in the direction of motion of the crossing lever system, and the point at which the spring takes effect on the crossing lever system may be changed by an adjustment system for different weights of drivers.
Driver's seats have to be able to undergo adjustment because of different weights of drivers using them, the general purpose being to see that for all drivers the distance moved against the spring or the range of spring effect is kept at the same value for all drivers, that is to say in a wide driver weight range. In the prior art this has generally simply been done by changing the pre-loading effect on the spring. The outcome of this, however, is that, when the seat is used by a driver who is on the heavy side, there is an undesired bouncing effect against the stops limiting motion of the seat. What would seem necessary in this respect would seem to be a seat design which makes it possible to get such an adjustment that for different driver weights in a given range (that is to say independently of the weight of a driver using the seat at a given time) that the inherent or natural frequency of the seat is kept at the same value, the natural frequency being given by the formula EQU f=1/(2.pi.).sqroot.(c/m)
wherein
f=natural frequency (sec..sup.-1) PA1 c=seat constant (N/m) PA1 m=mass (measured in kilograms).
The seat constant is generally dependent on the acting spring force. If, for adjustment of the seat to different weights of driver, only the spring pre-load is changed, there will generally only be a small change in the seat constant, or, at any rate, such change will not keep in step with the change in the weight of the drivers, the driver's weight counting as part of the mass m of the seat.